The present invention relates to coextruded, retortable thermoplastic film.
Pouches made from films or laminates, including polymers such as polyethylene or polypropylene, have found use in a variety of applications. For example, such pouches are used to hold low viscosity fluids (e.g., juice and soda), high viscosity fluids (e.g., condiments and sauces), fluid/solid mixtures (e.g., soups), gels, powders, and pulverulent materials. The benefit of such pouches lies, at least in part, in the fact that such pouches are easy to store prior to filling and produce very little waste when discarded. The pouches can be formed into a variety of sizes and shapes.
Pouches can be assembled from films, laminates, or web materials using vertical form-fill-seal (VFFS) machines. Such machines receive the film, laminate, or web material and manipulate the material to form the desired shape. For example, one or more films, laminates, and/or web materials can be folded and arranged to produce the desired shape. Once formed, the edges of the pouch are sealed and the pouch filled. Typically, the film, laminate, or web material has at least one heat seal layer or adhesive surface which enables the edges to be sealed by the application of heat. During the sealing process, a portion of at least one edge of the pouch is left unsealed until after the pouch is filled. The pouch is filled through the unsealed portion and the unsealed portion is then sealed. Alternatively, the pouch can be filled and the unsealed portion simultaneously closed in order to provide a sealed pouch with minimal headspace. The VFFS process is known to those of skill in the art, and described for example in U.S. Pat. No. 4, 589,247 (Tsuruta et al), incorporated herein by reference. A flowable product is introduced through a central, vertical fill tube to a formed tubular film having been sealed transversely at its lower end, and longitudinally. The pouch is then completed by sealing the upper end of the tubular segment, and severing the pouch from the tubular film above it.
Ethylene/vinyl alcohol copolymer (EVOH) has been used in high oxygen barrier VFFS applications to provide a package with a relatively long shelf life. A problem arises where the filled pouch is subjected to retort conditions. In such instances, film structures containing conventional EVOH undergo structural, cosmetic, and functional degradation.
The structural degradation is expressed as delamination, during the retort cycle, of the film layer containing the EVOH, from the adjacent layers of the film structure. This structural failure can lead to package failure if substantial enough.
The cosmetic degradation is expressed as film whitening during elevated temperatures, especially at high relative humidity. The aesthetic appearance of filled pouches is often important to the food processor as well as the final customer.
The functional degradation is expressed as a degradation in the oxygen barrier function of the EVOH, or put differently, an undesirable increase in oxygen transmission rate. This degradation is caused by the drop in barrier performance of EVOH at all relative humidities since the EVOH is actually damaged during retort. Voids will form in the EVOH during retorting and the barrier properties drop off significantly at all moisture levels. In most film structures containing EVOH, it is the EVOH that primarily controls oxygen transmission rate of the overall film. Therefore, a failure in the barrier properties of the EVOH translates into a failure of the barrier properties of the overall film. As a result, targeted shelf life is not achieved, and rapid product degradation can occur.
Because of these deficiencies, food products that require retorting are typically still packaged in rigid containers such as cans. These bulky containers create environmental issues about waste disposal. It would be beneficial to provide a film which can be filled and sealed on a form-fill-seal machine, and used in retort applications, without significant undesirable degradation during the retort cycle.
The invention is directed to a coextruded, retortable film for packaging of products such as liquid foods (soups, sauces, taco meat, etc.), with or without particulates, that require retort. The coextruded aspect of the invention is an important feature, because it would be beneficial to produce a coextruded film for use in retort VFFS applications. Laminating separate films or substrates together is time-consuming and costly.
In a first aspect, a multilayer, coextruded, retortable film comprises a core layer comprising an ethylene/vinyl alcohol copolymer; two intermediate layers, disposed on opposite surfaces of the core layer, comprising a polyamide; two adhesive layers, each disposed on a surface of the respective intermediate layer, comprising a polymeric adhesive; and two outer layers, each comprising a material selected from the group consisting of low density polyethylene, medium density polyethylene, high density polyethylene, ethylene/alpha olefin copolymer, propylene homopolymer, and propylene/alpha olefin copolymer.
In a second aspect, a method of packaging a food product comprises providing a multilayer coextruded retortable film, the film comprising a core layer comprising an ethylene/vinyl alcohol copolymer; two intermediate layers, disposed on opposite surfaces of the core layer, comprising a polyamide; two adhesive layers, each disposed on a surface of the respective intermediate layer, comprising a polymeric adhesive; and two outer layers, each comprising a material selected from the group consisting of low density polyethylene, medium density polyethylene, high density polyethylene, ethylene/alpha olefin copolymer, propylene homopolymer, and propylene/alpha olefin copolymer; forming the film into a tube in a vertical/form/fill/seal process; filling the tube with a food product; closing the tube to form a sealed pouch containing the food product; and subjecting the sealed pouch containing the food product to retort conditions.
In a third aspect, a package comprises a flowable retorted food product; and a pouch containing the food product, the pouch made from a multilayer coextruded retortable film comprising a core layer comprising an ethylene/vinyl alcohol copolymer; two intermediate layers, disposed on opposite surfaces of the core layer, comprising a polyamide; two adhesive layers, each disposed on a surface of the respective intermediate layer, comprising a polymeric adhesive; and two outer layers, each comprising a material selected from the group consisting of low density polyethylene, medium density polyethylene, high density polyethylene, ethylene/alpha olefin copolymer, propylene homopolymer, and propylene/alpha olefin copolymer.
In a fourth aspect, a multilayer, coextruded, retortable film comprises a first layer comprising a material selected from the group consisting of low density polyethylene, medium density polyethylene, high density polyethylene, ethylene/alpha olefin copolymer, propylene homopolymer, and propylene/alpha olefin copolymer; a second layer comprising a polymeric adhesive; a third layer comprising an ethylene/vinyl alcohol copolymer; a fourth layer comprising a polymeric adhesive; a fifth layer comprising a polyamide; a sixth layer comprising a polymeric adhesive; and a seventh layer comprising a material selected from the group consisting of low density polyethylene, medium density polyethylene, high density polyethylene, ethylene/alpha olefin copolymer, propylene homopolymer, and propylene/alpha olefin copolymer.
In a fifth aspect, a method of packaging a food product comprises providing a multilayer coextruded retortable film, the film comprising a first layer comprising a material selected from the group consisting of low density polyethylene, medium density polyethylene, high density polyethylene, ethylene/alpha olefin copolymer, propylene homopolymer, and propylene/alpha olefin copolymer; a second layer comprising a polymeric adhesive; a third layer comprising an ethylene/vinyl alcohol copolymer; a fourth layer comprising a polymeric adhesive; a fifth layer comprising a polyamide; a sixth layer comprising a polymeric adhesive; and a seventh layer comprising a material selected from the group consisting of low density polyethylene, medium density polyethylene, high density polyethylene, ethylene/alpha olefin copolymer, propylene homopolymer, and propylene/alpha olefin copolymer; forming the film into a tube in a vertical/form/fill/seal process; filling the tube with a food product; closing the tube to form a sealed pouch containing the food product; and subjecting the sealed pouch containing the food product to retort conditions.
In a sixth aspect, a package comprises a flowable retorted food product; and a pouch containing the food product, the pouch made from a multilayer coextruded retortable film comprising a first layer comprising a material selected from the group consisting of low density polyethylene, medium density polyethylene, high density polyethylene, ethylene/alpha olefin copolymer, propylene homopolymer, and propylene/alpha olefin copolymer; a second layer comprising a polymeric adhesive; a third layer comprising an ethylene/vinyl alcohol copolymer; a fourth layer comprising a polymeric adhesive; a fifth layer comprising a polyamide; a sixth layer comprising a polymeric adhesive; and a seventh layer comprising a material selected from the group consisting of low density polyethylene, medium density polyethylene, high density polyethylene, ethylene/alpha olefin copolymer, propylene homopolymer, and propylene/alpha olefin copolymer.
xe2x80x9cCore layerxe2x80x9d herein refers to the central layer of a film with an odd number of layers, or one of the two central layers of a film with an even number of layers.
xe2x80x9cEthylene/alpha-olefin copolymerxe2x80x9d (EAO) herein refers to copolymers of ethylene with one or more comonomers selected from C3 to C10 alpha-olefins such as propene, butene-1, hexene-1, octene-1, etc. in which the molecules of the copolymers comprise long polymer chains with relatively few side chain branches arising from the alpha-olefin which was reacted with ethylene. This molecular structure is to be contrasted with conventional high pressure low or medium density polyethylenes which are highly branched with respect to EAOs and which high pressure polyethylenes contain both long chain and short chain branches. EAO includes such heterogeneous materials as linear medium density polyethylene (LMDPE), linear low density polyethylene (LLDPE), and very low and ultra low density polyethylene (VLDPE and ULDPE), such as DOWLEX(trademark) or ATTANE(trademark) resins supplied by Dow, ESCORENE(trademark) or EXCEED(trademark) resins supplied by Exxon; as well as linear homogeneous ethylene/alpha olefin copolymers (HEAO) such as TAFMER(trademark) resins supplied by Mitsui Petrochemical Corporation, EXACT(trademark) resins supplied by Exxon, or long chain branched (HEAO) AFFINITY(trademark) resins supplied by the Dow Chemical Company, or ENGAGE(trademark) resins supplied by DuPont Dow Elastomers.
Films of the present invention are not solid state oriented, and exhibit a free shrink (ASTM D 2732-83) of less than 8%, preferably less than 5%, at 200xc2x0 in each of the longitudinal and transverse directions. These films are therefore non-heat shrinkable. Film of the present invention can exhibit a free shrink of less than 5% in the longitudinal direction, and less than 5% in the transverse direction, at 180xc2x0 F.
xe2x80x9cLow density polyethylenexe2x80x9d (LDPE) herein refers to a polyethylene having a density of between 0.915 and 0.925 grams per cubic centimeter.
xe2x80x9cMedium density polyethylenexe2x80x9d (MDPE) herein refers to a polyethylene having a density of between 0.926 and 0.939 grams per cubic centimeter.
xe2x80x9cHigh density polyethylenexe2x80x9d (HDPE) herein refers to a polyethylene having a density of between 0.94 and 0.965 grams per cubic centimeter.
xe2x80x9cIntermediatexe2x80x9d herein refers to a layer of a multi-layer film which is between an outer layer and an internal layer of the film.
xe2x80x9cInternal layerxe2x80x9d herein refers to a layer which is not an outer or surface layer, and is typically a central or core layer of a film.
xe2x80x9cLinear low density polyethylenexe2x80x9d (LLDPE) herein refers to polyethylene having a density between 0.917 and 0.925 grams per cubic centimeter, made by Zeigler/Natta catalysis.
xe2x80x9cLinear medium density polyethylenexe2x80x9d (LMDPE) herein refers to polyethylene having a density between 0.926 grams per cubic centimeter and 0.939 grams per cubic centimeter, made by Zeigler/Natta catalysis.
xe2x80x9cOuter layerxe2x80x9d herein refers to what is typically an outermost, usually surface layer or skin layer of a multi-layer film, although additional layers, coatings, and/or films can be adhered to it.
xe2x80x9cPolymerxe2x80x9d herein refers to homopolymer, copolymer, terpolymer, etc. xe2x80x9cCopolymerxe2x80x9d herein includes copolymer, terpolymer, etc.
xe2x80x9cRetortablexe2x80x9d herein refers to a film that can be formed into a pouch, filled with an oxygen sensitive product, and sealed, and subjected to sterilizing conditions of high temperature (between 250xc2x0 F. and 300xc2x0 F.), for a period of time of between 10 minutes and 60 minutes, in the presence of water, steam, or pressurized steam, without delamination of the EVOH layer from the adjacent layers of the film, or voiding of the EVOH and subsequent oxygen barrier loss. Typical retort conditions are 250xc2x0 F. for 30 minutes.
xe2x80x9cSolid state orientedxe2x80x9d herein refers to films obtained by coextrusion of the resins of the different layers to obtain a primary thick sheet or tube that is quickly cooled to a solid state, then reheated to an orientation temperature, then uniaxially or biaxially stretched using e.g. a trapped bubble or tenter frame process, and then rapidly cooled.
All compositional percentages used herein are presented on a xe2x80x9cby weightxe2x80x9d basis, unless designated otherwise.